Conventionally, an internal combustion engine is provided with a spark plug for ignition of an air-fuel mixture. The spark plug generally includes a center electrode, a ceramic insulator formed with an axial hole to retain the center electrode, a mount fitting (as a metal shell) surrounding a radial circumference of the ceramic insulator to retain the ceramic insulator and a ground electrode having one end portion fixed to the mount fitting and the other end portion facing the center electrode so as to define therebetween a spark gap in which a spark discharge occurs to ignite the air-fuel mixture.
It has recently been required to provide an engine intake valve or exhaust valve with a larger valve diameter for improvement in engine output performance and to secure a greater water jacket for improvement in engine cooling system. These requirements result in a smaller installation space of the spark plug in the engine so that the spark plug needs to be reduced in diameter. However, the insulation distance between the ceramic insulator and the mount fitting decreases with the diameter of the spark plug. It is thus likely that the spark plug will cause a so-called lateral spark, which flies from the center electrode to the mount fitting through the ceramic insulator, rather than a proper spark discharge within the spark gap. Further, it is likely that the spark plug will cause a so-called recess spark under a smoldering state as the insulation between the ceramic insulator and the mount fitting gets lowered due to the depositing of conductive carbon on a surface of the ceramic insulator. In such a case, it is necessary to raise a front end temperature of the ceramic insulator and burn off the carbon deposits from the ceramic insulator in order to secure the insulation between the ceramic insulator and the mount fitting as occasion demands.
In view of the foregoing, Patent Publication 1 proposes one type of spark plug that satisfies the following conditions: (X+0.3Y+Z)/G≧2, Y1 (mm)≧1, W/Z≧4 and 1.25≦Z (mm)≦1.55 where X is a distance from a front end portion of the ceramic insulator to the center electrode; Y is a creepage distance of a surface area of the ceramic insulator outside of the mount fitting; Y1 is an amount of protrusion of the ceramic insulator from the mount fitting; Z is an air pocket size; G is a spark gap size; and W is a length of a surface area of the ceramic insulator extending from a position corresponding to a front end face of the mount fitting to a position at which a distance between the ceramic insulator and the mount fitting is equal to the spark gap size G inside the mount fitting. By the above control of the respective component dimensions, the spark plug achieves a high ability to generate a spark discharge properly and stably within the spark gap under a non-smoldering state and to secure ignition performance even in the occurrence of a creeping discharge such as a lateral spark or a recess spark under a smoldering state. Patent Publication 1: Japanese Laid-Open Patent Publication No. 2005-116513
If the spark plug of Patent Publication 1 is applied to e.g. direct-injection engine in which smoldering is likely to occur, there is a problem of insufficient removal of the carbon deposits from the ceramic insulator whereby the spark plug cannot return to a state that provides adequate ignition performance. It is thus desired to develop a technique for burning off the carbon deposits from the ceramic insulator quickly in order to return the spark plug from a smoldering state to a normal operating state and thereby secure ignition performance.